The class WatchedItemStore has a function clearUserWatchedItems that is used to removed a user's watched items from the store by deleting related rows from watchlist table. The function needs to delete the items from the watchlist_expiry table as well.
|mediawiki/core||master||+74 -6||Remove expired items in WatchedItemStore::clearUserWatchedItems|
@dom_walden This code only affects a test in WatchedItemStoreIntegrationTest.php (https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/097c6b575d44848f6531a84c5556a7ca2ab507a7/tests/phpunit/includes/watcheditem/WatchedItemStoreIntegrationTest.php#198).
EditWatchlist/clear and EditWatchlist/raw have their own clearUserWatchedItems function, which was indeed tested in T245224